Many reproduction methods are only capable of reproducing a small number of stable image tones. For example, offset printing is only capable of printing two stable tone values i.e. deposit ink or not. In order to reproduce images having continuous tones, a halftoning or screening technique is used. In the graphic arts environment, halftoning techniques convert density values of tints and images into a geometric distribution of binary dots that can be printed. The eye is not able to see the individual halftone dots, and only sees the corresponding "spatially integrated" density value. In a more general context, halftoning techniques can be seen as methods to convert "low spatial, high tonal resolution information" into an equivalent of "high spatial, low tonal resolution information". (The qualifiers "low" and "high" have to be seen on a relative scale in this context).
Two main classes of halftoning techniques have been described for use in the graphic arts field. These two techniques are known as "amplitude modulation" and "frequency modulation" screening. In amplitude modulation screening the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid. By varying the size of the halftone dots, the different tones of images can be simulated. Consequently, this technique can also be called "dot-size modulation screening". In frequency modulation screening the distance between the halftone dots is modulated rather then their size, and can also be referred to as "dot-position modulation screening". This technique, although well known in the field of low resolution plain paper printers, has not obtained much attention for offset printing and other high end printing methods, probably because of the disadvantages to be discussed below.
Both classes of halftoning techniques are used in combination with a digital film recorder. A typical digital film recorder employs a scanning laser beam that exposes a photosensitive material at high resolution. The "grid" that defines the resolution at which the laser beam can be switched on or off, usually has an element size in the range of 01/1800 of an inch. The photosensitive material can be a photographic film from which a printing plate is later prepared by means of photomechanical techniques. The smallest addressable unit on a recorder is often called a "micro dot", "recorder element", or "rel". Its size is referred to as the recorder "pitch". As illustrated in FIG. 1A a dot-size modulated halftone dot is made up of a clustered set of recorder elements, while in FIG. 1B frequency-modulation halftone dots consist of individual recording elements.
The most important characteristics of a screening or halftoning technique for faithfully reproducing continuous tone information include:
1) The image rendering characteristics, more specifically the capability of the technique to render spatial detail in the original image content without the introduction of artifacts such as moire, textures and noise, as well as the capability to render a full range of tones; PA0 2) The photomechanical characteristics of the halftone dots produced by the method, which determine how consistently halftone dots can be recorded, copied or duplicated in the different steps of the photomechanical preparation of the printing plates; and, PA0 3) The behavior of the halftones on an offset printing press.
The two classes of halftoning techniques, each with some of their variants, will now be reviewed in the light of the above characteristics, and their advantages and disadvantages will be discussed